If you are sure that you know the answers to the other two questions then you might be able to change them by going to https://appleid.apple.com/ and clicking on 'Manage your Apple ID' on the right-hand side of that page and log into your account. Then click on 'Password and Security' on the left-hand side of that page - you should be asked 2 of your 3 questions, so you may be asked the 2 that you know.
Otherwise, if you have a rescue email address (which is not the same thing as an alternate email address) set up on your account then go to the same screen i.e. go to https://appleid.apple.com/ , click 'Manage your Apple ID' on the right-hand side of that page and log into your account, and then click on 'Password and Security' on the left-hand side of that page. There should be an option to send security question reset info to your rescue email address.
If you don't have a rescue email address then see if the instructions on this user tip helps : https://discussions.apple.com/docs/DOC-4551